A frequency domain telecommunications transmission such as an OFDM transmission typically comprises a series of symbols of fixed duration, or length, each containing data modulated onto a number of carriers, or tones, of different frequencies. A typical symbol may be of length 250 microseconds, in which case the carriers are spaced at frequency intervals of 4 kHz to ensure orthogonality between the carriers. Depending on the frequency bandwidth available, the symbol may contain, for example, 1000 carriers. If each carrier is phase modulated using quadrature phase shift keying (QPSK), each carrier can carry two bits of data. Each symbol then carries 2000 bits.
In the sequence of symbols, adjacent symbols are separated by a few microseconds, such as 10 microseconds. FIG. 1 illustrates a sequence of symbols 20 separated by gaps 22.
It is known to transport telecommunications signals using a frequency domain transmission system in a plurality of separate frequency bands at the same time. The frequency bands may be non-contiguous. Such transmission can be achieved using a frequency domain modulation scheme such as OFDM or coded OFDM (COFDM) and is desirable in transmission media which carry other signals in predetermined frequency bands or suffer from noise in particular frequency bands.
Examples of such media are the use of powerlines (PL) for transporting telecommunications data to and from subscribers, cable television (CATV) systems and fixed wireless access (FWA) systems.
Use of OFDM for signal transmission in PL media is known from U.S. 09/419,209, which is incorporated herein by reference. U.S. 09/419,209 also describes the use of OFDM for signal transmission in more than one frequency band, as follows.
One of the problems with using power lines as a communications medium is that they are subject to noise and interference. A first type of noise is due to cables picking up radio signals such as broadcast AM radio signals and amateur radio band transmissions. Overhead cables are particularly prone to this type of noise. A second type of electrical noise is due to electrical equipment coupled to the power lines. Electric motors, thermostats and gas discharge lighting are particularly prone to generating noise. Noise propagates along the power lines and combines with communications signals. The level of noise can be high enough, and persist for long enough, to corrupt communications signals.
CATV and FWA suffer similar problems, though not necessarily from the same source. For example, in CATV noise can result from ingress at the consumer connection and inter-modulation products from the TV carriers.
FIG. 2 shows typical background noise on an underground power line across the frequency band 0-10 MHz. It is known to be advantageous to transmit communications signals within the frequency bands 2.2-3.5 MHz (PLT1) and 4.2-5.8 Mz (PLT2) because these bands fall between the medium wave and short wave bands used for broadcast radio transmissions and avoid the radio amateur band at 3.5-3.8 MHZ. There is a reduced level of background noise in these bands and the radiation of power line communications signals in this frequency band causes minimum interference with radio receiver equipment at subscriber premises. Other frequency bands in the range of, for example, 2-30 MHz can be used although it is preferred to use the lower frequencies because attenuation over the distribution cables is lower.
The use of OFDM provides flexibility to fit into non-uniform and non-contiguous frequency allocations, while maintaining reasonable spectral efficiency. As illustrated in FIG. 3, this results from the intrinsic nature of an OFDM signal, which is composed of a large number of simultaneously transmitted sub-carriers 24 which are staggered in frequency each individually occupying a low bandwidth (a reduced number of carriers is shown schematically in FIG. 3). Portions 26, 28 of a symbol containing such carriers can be placed in non-contiguous bands between frequency bands 30 in which noise or other signals are present.
The scheme's flexibility comes about from the ability to designate which carriers within an available frequency range are to be activated and which are not. Regarding spectral efficiency, the signal composition results in an intrinsic spectrum fall-out outside the active bandwidth commensurate with the bandwidth of each carrier rather than with the total spectrum width. Thus relatively low excess bandwidths can be achieved.
Therefore, the spectral attributes of OFDM represent a major advantage in favour of its selection for use in power line telecommunication systems.
Symbol Synchronisation
To decode a FDM symbol, a receiver uses a Fast Fourier Transform (FFT) to convert the symbol from the frequency domain to the time domain, in order to recover the transmitted data. To use an FFT to decode a symbol and extract data using an FFT processor at a receiver, it is necessary to know the time of arrival of the symbol at the receiver because the FFT can only be performed once, and the samples input to the FFT processor (after analogue to digital (A/D) conversation of the received symbol) must be taken only during the symbol period. The acquisition of symbol arrival time information to sufficient accuracy to perform an FFT (accuracy of approximately 5 microseconds on a 250 microsecond symbol is required) can be termed coarse synchronisation. As a second function, it is also necessary to adjust the sampling clock in the receiver to match the transmitter clock, in order to allow demodulation of long sequences of symbols containing phase modulated data without re-adjustment of a phase reference at the receiver. This can be termed fine synchronisation.
Conventional Coarse Synchronisation
This is achieved either by a front/back correlation method or by amplitude envelope detection.
In a front/back correlation method, as conventionally used in Digital Video Broadcasting (DVB), transmitted symbols are elongated during a synchronisation period by copying a portion of the frequency domain signal from the front, or start, of each symbol onto the end of the same symbol. Because there is an integral number of cycles of each carrier frequency during the original symbol period, the transition from the end of the original symbol to the elongating portion is phase continuous, and so does not corrupt the frequency spectrum. FIG. 4 illustrates schematically an extended symbol 10 in which the start portion 12 of a standard symbol 14 has been duplicated at the end portion 16 of the extended symbol.
At the receiver, the received signal is split into two portions, and one portion is delayed by the known normal symbol period. Correlation of the received and delayed data streams produces a peak correlation signal when the end portion of an extended symbol in the received data stream coincides with the front portion of the same symbol in the delayed data stream. The correlation peak provides a timing signal allowing the receiver to synchronise to the symbol timing.
The front/back correlation method of synchronisation is disadvantageously vulnerable to out of band carriers or interference in systems such as the media described above, where different signals may be carried in different frequency bands or significant levels of interference in certain frequency bands may be expected. In practice, the correlation process, using for example an 11.25 microsecond window, provides about 20 dB processing gain, depending on the autocorrelation function of the interfering signal. This gain may not be sufficient to synchronise to received symbols in the presence of significant out of band noise. In addition, the receiver uses an FFT as a filter in the data decoding phase, after synchronisation, which rejects out of band carriers. Typically the FFT has a stop band rejection of about 50 dB. The filtering action of the FFT is therefore much higher than that of the correlation process, potentially leading to the undesirable situation that the synchronisation process could be less robust than the data decoding process.
The amplitude envelope detection method of synchronisation involves the direct detection of symbols as received, as pulses of known length in the received signal itself. This system is usually less robust than the front/back correlation method, particularly in dual mode digital/analogue systems. In such cases, analogue transmissions carried in frequency bands outside the frequency band carrying digital symbols may generate signal bursts of variable length, which may include the length of the digital modem symbol. This may lead to false synchronisation timing in the amplitude envelope detection method.